Spermatogenesis, hormonal fluctuations, and testicular gene expression changes in male blue catfish, Ictalurus furcatus, throughout pubescent and early adult stages of development

Male blue catfish (Ictalurus furcatus) mature at 4-7 years and cannot be hand-stripped, making sperm collection a lethal procedure. Sperm are also highly variable in quality/quantity. We monitored blue catfish males for 34 months (2.3 to 4.9 years old; y/o) to quantify changes in body morphometrics, plasma androgens, osmolality, ions, testicular development, and gene expression. Body weight and length of fish ranged from 0.6 ± 0.2 to 3.2 ± 0.2 kg and 35.2 ± 2.1 to 58.9 ± 1.9 cm, respectively. Testes weight and gonadosomatic index (GSI) were highly variable within/between sampling points. Maximum testes weight was 6.95 g (4.7 y/o) and GSI was 39.9% (3.7 y/o). Age-associated changes in testicular development were significant, with highest spermatogenic maturity values (0.31-0.73) reported for 3.11 to 4.9 y/o fish and lowest for 2.3 (0.13) and 2.11 (0.12) y/o fish. Spermatozoa were observed in 3.3 and 4.2 to 4.9 y/o fish. The highest ratio of spermatozoa was detected at 4.2 y/o (70%). Androgens spiked at 3.11 y/o, coinciding with reproductive season. An increase in osmolality (mOsm/kg) was observed for 3.11 to 4.5 y/o fish, then decreased. K⁺, Na⁺, and Cl^- were highest ions throughout development. Transcriptomic analysis revealed 6,317 differentially expressed genes between 2.9, 3.1, 3.9, and 4.2 y/o fish. Most genes were differentially expressed in 3.1 (2,348) and 4.2 (2,689) y/o fish, when compared to 2.9 y/o. Genes related to spermatogenesis and sperm development (cfap61, hsf5, rsph9, dmrt2a, LOC128611646, usp21, gstt1b) were differentially expressed based on age and season. These findings improve our understanding of the reproductive process for an important farm animal.